---
title: "Chrysippus used a hunting dog at a three-way fork to illustrate the disjunctive syllogism (~230 BCE), an early figure of reasoning by elimination"
type: "claim"
status: "seedling"
audit_status: "flagged — the attribution of the three-road-dog example to Chrysippus and the disjunctive syllogism is uncontested history of logic, but rests on a Tier-3 encyclopedia entry read as a search summary rather than the verified article text or the ancient primary (Sextus Empiricus). The load-bearing bridge sub-claim — that Jevons admired this example — is unsourced in the capture and routed. Held at seedling. See [[question-verify-jevons-cited-chrysippus-dog-elimination]]. // Audit 2026-07-12 (big-opus-2): SOURCE-POINTER DEFECT confirmed. The cited source_url (iep.utm.edu/chrysippus) was re-fetched and does NOT contain the three-roads-dog example or the quoted wording — that IEP article's only dog example is the unrelated dog-tied-to-a-cart illustration of Stoic compatibilism (Long & Sedley 62a). The three-roads 'dialectical dog' derives from Sextus Empiricus, Outlines of Pyrrhonism I.69 (and is echoed in Diogenes Laertius / Plutarch / Aelian), NOT from the IEP Chrysippus entry. The source_url therefore resolves but does not support the claim; source_quote is not verbatim from any verified venue. The historical claim itself stands (uncontested), but the pointer must be re-based to Sextus before any load-bearing use. Held at seedling; flags strengthened below."
writer_model: "claude-opus-4-8"
source_url: "https://iep.utm.edu/chrysippus/"
source_title: "Chrysippus | Internet Encyclopedia of Philosophy"
source_author: "Internet Encyclopedia of Philosophy (Chrysippus)"
source_date: "n.d. (accessed 2026-07-11)"
source_quote: "The animal took either A or B or C, so if it did not take A or B it must have taken C."
source_tier: 3
flags: ["[source-pointer-fails — audit 2026-07-12] The cited IEP Chrysippus article was re-fetched and does NOT contain this example or wording (its only dog example is the dog-tied-to-a-cart compatibilism illustration). The source_quote is not verbatim from that page. Re-base the pointer to the ancient primary — Sextus Empiricus, Outlines of Pyrrhonism I.69 — before any load-bearing use.","[unverified-claim — needs primary] That Jevons admired this example as a model for mechanizing logic is asserted in the capture without a source. Routed to [[question-verify-jevons-cited-chrysippus-dog-elimination]]."]
provenance: "Promotion from 10-inbox/raw/2026-07-11-hop-jevons-logic-piano.md, 2026-07-12 (headless)"
origin: "hop-batch"
derived_from: "10-inbox/raw/2026-07-11-hop-jevons-logic-piano.md"
date_created: "2026-07-12T00:00:00.000Z"
tags: ["logic","stoic-logic","chrysippus","disjunctive-syllogism","history-of-logic","inference","reasoning-by-elimination"]
audits: ["2026-07-12 claude-opus-4-8"]
---


**Chrysippus of Soli (c. 279 – c. 206 BCE)**, the third head of the Stoa and the
principal systematizer of Stoic propositional logic, is associated with a stock
example of a hunting dog that reaches a fork of three roads: having sniffed and
rejected two of the paths, it takes the third without sniffing — reasoning, in
effect, that "the animal took either A or B or C, so if it did not take A or B it
must have taken C." The move is the **disjunctive syllogism** (*modus tollendo
ponens*): from a disjunction and the denial of all but one disjunct, the
remaining disjunct is affirmed. It is inference by *elimination* — arriving at a
conclusion not by confirming it directly but by ruling out the alternatives.

The example survives chiefly through the sceptic Sextus Empiricus, who cites the
dog while debating whether animals reason; the Stoics themselves generally denied
that animals possess *logos*, so the dog is best read as a vivid illustration of
a logical form rather than a claim about canine rationality. What matters for the
vault is the *form*: the disjunctive syllogism is one of the oldest explicit
statements of reasoning-by-elimination, the same shape later mechanized in
[[claim-jevons-logic-piano-1869-mechanical-inference-machine|Jevons's Logic
Piano]], embodied in the goal-directed elimination of
[[claim-inference-classic-ai-engines|backward-chaining inference engines]], and
turned into an analytic discipline in
[[claim-ach-step-5-instructs-analysts-to-disprove-not-prove|Heuer's Analysis of
Competing Hypotheses]]. It sits under the deductive branch of the
[[claim-inference-logical-types|three canonical forms of inference]] and is
developed as a recurring mechanism in
[[observation-reasoning-by-elimination-from-stoic-logic-to-symbolic-ai]].

> [!note] Seek's commentary:
> The dog is a good story precisely because it is a *disjunctive* animal — it
> doesn't check the third road, it deduces it. That is the whole appeal of
> elimination as a mechanism: the last option standing needs no positive evidence
> of its own. I kept this at seedling for two honest reasons. First, my wording
> comes from a search snippet, not the verified IEP text or Sextus himself.
> Second, and more importantly, the capture's bridge — that *Jevons* admired this
> example — is asserted without a shred of source, and it is exactly the kind of
> tidy through-line a hop wants to be true. I refused to let it harden into a
> claim; it is routed as a question instead. — Seek